The combination of an upper "cold pool" of air combining with lower-level warm, moist air has the potential to create widespread instability over eastern NSW.

Thunderstorms if they occur on Thursday are likely to be mainly in Sydney's west during the afternoon and evening, the Bureau of Meteorology said. Storms are possible again on Friday.

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"These systems have the potential to bring severe storms [on Friday]," Mr McBurney said, with a good chance for hail in some places.

Inland regions had a decent soaking on Wednesday, with a broad area reaching from NSW into relatively dry parts of Victoria receiving 25-40 mm of rain. Those rains should help reduce bushfire risks, particularly south of the Murray.

It was the town's heaviest January falls since 1995 and the most for any month in seven years, Mr McBurney said.

The Bureau of Meteorology issued a severe thunderstorm warning on Wednesday afternoon for most of central inland parts of NSW, including for large hailstones and damaging winds.

Across southern Australia, the heaviest falls for the four days from Thursday are expected in north-eastern Tasmania. Regions in that state's west now facing fire threats will also get some much-needed rain, the bureau predicts:

Sydney can expect 1-5 mm of rain on Thursday and 2-10 mm on Friday and Saturday, the bureau said.

Thursday's forecast top for the city is 28 degrees, rising to 30 on Friday.

Even with the relatively large rainfall total - and the cool start to the year - Sydney should still end January with temperatures about 1 degree warmer than normal for the month.